Dual tower regenerative desiccant dryers are conventionally used to separate a gas fraction from a gas mixture. For example, such dryers are commonly used to remove moisture from compressed air to provide dry compressed air at dew points below the freezing point of water or to reduce the moisture content of compressed air for use in critical process applications. With known dryers, dew points as low as -150.degree. F. can be achieved.
Known dryers are able to continuously dry compressed air by using two identical towers, each containing a desiccant bed. While one tower is on-stream and is drying compressed air by passing the air flow through the desiccant bed therein, the other tower is off-stream and its desiccant bed is being regenerated (i.e. reactivated or dryed out). The towers are alternated on and off stream by a suitable control system so that a dry desiccant bed is always in contact with the stream of wet compressed air passing through the dryer, thus resulting in a continuous supply of dry compressed air downstream from the dryer.
Known regenerative desiccant dryers employ a variety of methods for desiccant regeneration. In heatless or pressure swing dryers, the desiccant is regenerated by expanding a small portion of the dry air product from the on-stream tower to near atmospheric pressure and passing the expanded air through the moisture laden desiccant bed in the tower being regenerated. The heat of adsorption stored in the moisture laden desiccant bed is utilized, in conjunction with the expanded air portion, to extract (or desorb) moisture from the desiccant bed. By contrast, heated dryers require external energy sources to provide the heat of desorption and a carrier gas to remove the desorbed water from the system. The present invention concerns a heatless or pressure swing type of regenerative desiccant dryer.
Such pressure swing desiccant dryers have been known for many years. For example, U.S. Pat. No. 2,944,627 discloses one such pressure swing dryer. Because such apparatus is so well known, it is believed that inclusion of detailed description of such known dryers herein is unnecessary for an understanding of the present invention. The reader is referred to the above cited U.S. Pat. No. 2,944,627, or any other of the several prior patents cited hereinbelow, for a full description of prior dual tower pressure swing type regenerative desiccant dryers.
As noted above the desiccant beds in the drying towers of such a dryer are regenerated by exposure thereof to an expanded portion of the compressed air product. For example approximately 15% (at 100 psig) of the dried compressed air product may be expanded to near atmospheric pressure and then passed as a purge air stream through the tower to be regenerated. Due to this "swing" in pressure of the dried compressed air, the expanded air becomes extremely dry, and passage of the dry purge air through a tower causes moisture to desorb from the desiccant, whereupon the desorbed moisture is carried by the purge air stream out of the dryer. To alternately purge moisture from one and then the other of the dual towers of such a dryer apparatus, well known piping and valving networks are employed, also as disclosed in the above cited U.S. Pat. No. 2,944,627 and other prior patents cited hereinbelow.
In general, pressure swing dryers which operate on a fixed timing cycle with a fixed purge flow rate can perform very well. If the throughput flow of compressed air remains constant at maximum design flow, there generally is no incentive or need to modify the operating cycle of such dryers. In many industrial environments, however, compressed air demand and environmental conditions do not remain constant but vary over time. With such variations, the condition of compressed air entering the dryer can vary widely, exhibiting variations in flow rate, pressure, temperature and dew point. Furthermore, the energy consumption of such dryers can be significant indeed, especially for larger capacity dryers. Efficient control of dryer operation can effectively minimize dryer operating costs.
More specifically, significant economies may be realized if a drying system can be monitered and the regeneration cycled adjusted to reduce purge requirements. For example, a dryer with a 1000 SCFM (standard cubic feet per minute) inlet flow capacity would require approximately 150 SCFM of purge air (15% of inlet flow). To supply this purge air at a cost of, for example, 20.cent. per 1000 standard cubic feet for one year of continuous operation, the total cost would be $15,768.00. If the annual compressed air demand were less than system capacity (e.g. demand only 50% of capacity) a corresponding reduction in regeneration requirements is also realized and consequently half the cost of purge air for regeneration ($7,884.00) could be saved by use of a demand control system which effectively regulates the use of purge air.
There are known in the prior art a variety of demand control systems for pressure swing type desiccant dryers. For example, U.S. Pat. No. 3,448,561 discloses a system for controlling the frequency of regeneration for either a heatless or heated desiccant dryer by use of a hygrometer system which samples air from the drying desiccant bed and measures its moisture content. When the moisture content is reduced to a predetermined value, the regenerated tower is placed on line as the other tower is switched to the regeneration mode.
U.S. Pat. No. 4,247,311 discloses a pressure swing dryer system wherein a microprocesser is used in conjunction with transducers which sense moisture to moniter the advance of the moisture front through the drying towers. This information is used to initiate the switching of the towers between the drying and regeneration modes. According to this patent, the system operates on a fixed cycle with the signal from the moisture sensing transducer being utilized to control the frequency of regeneration cycles (i.e. for 50% demand, a drying tower would be regenerated during alternate cycles, while remaining pressurized in a standby mode during the intervening cycles). Other known demand control systems include the following. Deltech Engineering, Inc. has introduced a demand control system for a pressure swing dryer which operates on a fixed ten minute cycle and takes measurements of inlet temperature, inlet pressure, and outlet mass flow rate for input into a microprocessor which then calculates and controls purge flow rate and purge time for the regeneration cycles.
Zurn Industries, Inc. offers a demand control system for pressure swing dryers which utilizes moisture sensors located near the wet air inlet in the desiccant beds. The sensors transmit information on inlet air moisture content to a microprocessor which determines and controls the duration of the regeneration cycle.
A technical manual entitled "A Heaterless Dryer Design Manual" by Ed Peacock suggests a demand control system based on monitoring the temperature rise across the desiccant bed by the use of temperature probes in the inlet and outlet air streams.
The prior art of demand control systems for pressure swing desiccant dryers generally has required incorporation of expensive, sensitive control components which must be continually maintained and recalibrated to provide consistent drying tower switchover. For example, the above described demand control system based on temperature rise across the desiccant bed requires, as noted, temperature transducers in the inlet and outlet piping or near the inlet and outlet ends of the drying towers. Since such a system depends on accurate sensing of inlet and outlet temperatures, small deviations of either the inlet or outlet temperature transducer from proper calibration values can result in delayed or premature tower switchover between the drying and regeneration cycles. In addition, in such a system small variations in inlet air temperature may provoke undesired variation in the regeneration cycle.